The present invention relates to a double steel pipe structural member for use in a truss or brace structure and, more particularly, to a structural member with high resistance to elastic and plastic buckling and which is capable of large controlled axial plastic deformation under high compressive loads.
In constructing a large structure by using a large number of structural members comprising long steel pipes, connection of the structural members with nodes enables a space truss structure. In order to join radially a plurality of structural members to spherical nodes provided with several fastening portions a joint device equipped with a fastening bolt, movable in the axial direction of the structural member, is used.
Such joint devices which tightly connect steel pipes to nodes are disclosed in U.S. Pat. No. 4,872,779 and U.S. Pat. No. 5,141,351.
Each device substantially comprises a fastening bolt having a threaded part engaged with a screw hole of a connector node and a sleeve slidably engaged with a polygonal boss formed on the intermediate part of the bolt. Rotation of the sleeve advances the fastening bolt, mounted in the end of the structural member, toward the node.
When a truss structure assembled by structural members is covered with heavy snow or is present during a large earthquake, axial compressive load is communicated to any steel pipe member through the nodes joined to both ends thereof. A large compressive load promotes elastic and plastic buckling of structural members, and in general, the strength of a single steel pipe decreases immediately as shown by a double dotted chain line A in FIG. 11 and, thus, the truss structure will quickly collapse.
U.S. Pat. No. 4,281,487 discloses a double steel pipe member to control buckling of elongated structural members and to shorten the overall length of the structural member by means of the predetermined plastic axial deformation of the external pipe after an axial compressive force exceeds a yield strength thereof. Both the external and internal steel pipes can oppose the axial compressive force which successively acts on the structural member and can promote an earthquake-proof character of the truss structure.
A structural member similar to the above is disclosed in GB 2,248,862 A, which is provided with an external pipe 44 and an internal pipe 55 inserted therein as seen in FIG. 10. Such a member comprising a double steel pipe is joined to nodes 3 using a joint device 51 installed on end cap 66 closing the end of the external pipe 44.
The clearance between an outer surface of the internal pipe 55 and an inner surface of the external pipe 44 is made as small as possible and the length of space 11 between an end face of the internal pipe 55 and an inner face of the end cap 66 is defined as one-half of the predetermined compressive deformation .beta. allowable for the external pipe 44.
Even when an axial compressive load P acts on the external pipe 44, the external pipe 44 is prevented from deforming in the direction orthogonal to the longitudinal axis 44m thereof by means of the internal pipe 55 behaving as a bending resistant pipe, whereby, the elastic and plastic deformation of the pipe 44 is suppressed. The external pipe 44 is plastically shortened in the direction of the longitudinal axis thereof only along the outer surface of the internal pipe 55.
On the other hand, once the external pipe 44 is largely compressed, both ends of the internal pipe 55 will contact the inner faces of the end cap 66 as shown by double dotted chain lines. Thereafter, a resultant force obtained by adding the strength of the internal pipe 55 to the residual strength of the external pipe 44 resists the succeeding axial compressive force acting thereon so that the double steel pipe never immediately deforms.
Since the afore-mentioned double pipe initially has no resistant members within the space 11, an initial plastic deformation by means of the axial compressive force occurs close to the end cap 66. The deformation of the end portion of the external pipe 44 becomes unstable and axially unsymmetrical. Therefore, the external pipe 44 is forced to bend near the node 3. The appearance of partial bending of the external pipe hinders the introduction of axial force only to the double steel pipe in a truss structure.
In addition, unstable deformation of the external pipe 44 never achieves a desired shrinkage in the axial direction equal to the predetermined compressive deformation .beta. allowable therefor. Consequently, the end face of the external pipe 55 can not contact the inner face of end cap 66, which means the strength of the internal pipe 55 is of no utility as shown by broken line B in FIG. 11 indicating an immediate decrease of the strength of the steel pipe just after the beginning of the plastic deformation.
An object of the present invention is to propose a double steel pipe structural member to suppress elastic and plastic deformation of an external pipe when an axial compressive load is acting thereon.
Another object is to prevent the external pipe from bending locally against an axial compressive force more than the yield strength thereof, to facilitate a large compressive deformation of the external pipe in the axial direction thereof, and to enable the structural member to withstand the axial compressive force by means of maintaining the plastic deformation capacity when the axial force acting on the steel pipe becomes more than the yield load thereof, thus, preventing a truss structure from immediately collapsing.